This invention relates generally to structures for concentrating light, and, more specifically, to arrays of microlenses for concentrating light onto a pixilated array.
There are numerous applications in digital imaging where millions of tiny light-collecting elements or microlenses are densely packed in a pixilated array. These arrays appear in imaging devices such as digital cameras, image scanners, and potentially in solar cells. The necessary presence of micro-electronic devices and wires on the front surface of the detectors limits the area available for light-sensitive regions. When properly designed, microlenses situated on each element direct as much light as possible away from the insensitive regions and onto the photo-detector elements. The present invention rejects conventional wisdom and the state of the art concerning how these lenses should be shaped, and suggests shapes that are both easy to fabricate and can be significantly more energy efficient.
Prior art microlens designs imitate traditional large lenses in form and intended function, as shown in prior art FIGS. 1A and 1B. Such prior art microlens designs are based on plano-convex lenses formed from a solid block of transparent material sitting above an array of light-sensitive detector elements. The prior art design in FIG. 1A shows the microlenses positioned only above the detector elements. The prior art design in FIG. 1B shows the microlenses covering the entire available area with spherically-shaped segments in order to maximize the collection and concentration of light.
Such microlenses 110, 120 approximate plano-convex lenses with spherical-cap shapes formed at the surface of a thick, transparent material 115, 125 that fills the space above light-sensitive detector elements 130, 140. FIG. 1C is a cross-section cut of FIG. 1B as viewed along the arrows C-C. According to classical optics, where the index of refraction of the transparent material is n (>1), and the radius of the spherical cap is R, incident light from a faraway bright object is focused inside the thickness of the lens material, with a focal length f. Calculation off follows the so-called Lensmaker's Equation, which, in this simple case reduces tof=Rn/(n−1)Since n is greater than 1, the focal length f will be greater than the R, and is usually several times R.
However, miniaturization complicates the physics of light focusing considerably. Where the sizes of the microlens elements reach the micron scale, the rules of classical “ray-tracing” no longer apply. The pixel sizes of interest (i.e., the array periods) are below 5 μm, and some designs may extend below 1.0 μm. The wavelengths of visible light span the range 0.4 to 0.7 μm, meaning that the microlens widths are on par with a single wavelength, and may be up to several wavelengths wide, at most. At this small size, the physics of light propagation is dominated by diffraction.
Detailed finite-difference time-domain (FDTD) numerical simulations have been used to model the vector electromagnetic field as the light propagates through microlens systems. As shown in prior art FIG. 2, the calculations show that the traditional hemispherical lens design is not optimized for concentrating light onto a detector. The microlens geometry is shown in the column at left, with light incident from above and a simplistic representation of light focusing shown in light gray. Calculations are made for a range of incident wavelengths from 400 to 700 nm, labeled at the top of each column. The grayscale images represent the light intensity in a plane that contains the apex of the microlens; each calculation is individually scaled, and the linear intensity scaling is shown at right. For the specific case shown (R=1.44, n=1.3), the focal length predicted by the Lensmaker's Equation isf=Rn/(n−1)=6.24 μm,independent of wavelength. The simulations show that the depth where the light concentration is greatest occurs significantly above the focal length predicted by the Lensmaker's Equation. Furthermore, there is a significant chromatic dependence, meaning that different wavelengths of light (i.e. different colors) are focused to different depths within the material. Therefore, designs based on classical ray-tracing considerations lead designers to place light-sensitive detectors in regions with relatively low light concentrations (i.e., power densities).
An additional consideration is cross-talk, or the tendency for light incident on one lens element to fall onto the detector element or elements belonging to adjacent pixels. The larger the longitudinal distance between the microlens and the detector, the higher the chance that the design will allow cross-talk to occur. Such considerations are excluded from the simple, ray-tracing model.
New light-focusing structures that take into account the physics of light propagation at this very small scale are clearly needed.